In a density concurrent matrix, the relationship of densities between two pixels in an image is stored, which are remote from each other by predetermined distance. The distance is usually defined as .delta. (x,y), and it means coordinate differences x in x direction and coordinate difference y in y direction. Each element in the density concurrent matrix represents a number of couples of densities, that is, on the d.sub.1 th column and d.sub.2 th row, the number of couples of densities d.sub.1 and d.sub.2 is stored. A matrix is proposed by Haralick, in which the summation of the number of couples of densities d.sub.1 and d.sub.2, as well as the number of couples of densities d.sub.2 and d.sub.1 are stored in the same column and same row.
As far as the inventor knows, there has been disclosed no concrete method for generating the density concurrent matrix. If all couples in an image are compared in density, N comparisons are necessary for an image of N pixels. When the image size is 512.times.512, more than 260 thousands times comparation is necessary. It takes several minutes for a 16-bit personal computer to execute the comparisons.
Characteristics parameters; contrast, second moment correlation etc.; are usually calculated from the density concurrent matrix. There is disclosed no concrete method for calculating such parameters.